The present invention relates to signal converting circuits and, more particularly, to a circuit for converting signals from a CMOS (complementary metal-oxide-semiconductor) circuit into signals for an ECL (emitter-coupled logic) circuit.
Computer systems today use circuits implemented in different hardware technology. For example, it is not uncommon to find in one computer both CMOS and ECL circuits. CMOS is often used in very large chips having many transistors (in excess of 100,000) because of the suitability of CMOS in achieving the power dissipation requirements of that many transistors. Other smaller, but faster, chips are normally implemented in ECL.
One problem with having circuits in one system implemented in different hardware technologies is that the same logic levels of the circuit signals are represented by different voltages. For example, in a CMOS circuit, a logic level "1" will typically be represented by ground (0 V) and a logic level "0" will typically be represented by -5 V. An ECL circuit, on the other hand, will typically have a logic level "1" represented by -0.98 V and logic level "0" represented by -1.6 V. It thus becomes necessary to provide a signal converter or interface when the signals from a CMOS circuit are provided to an ECL circuit.
Circuits have been designed for converting CMOS logic signals into ECL logic signals. For example in U.S. Pat. No. 4,704,549, entitled "CMOS to ECL Converter-Buffer", which is assigned to the same assignee as herein, there is shown a circuit having MOS transistors for converting CMOS logic signals to ECL logic signals. One drawback of this circuit is the fact that the transistor that provides the ECL logic level signals at the output of the circuit requires a voltage in addition to the normal voltages (V.sub.DD and V.sub.SS) used to power CMOS devices. An external power source, which increases the cost of the circuit, is necessary to provide the additional voltage. Another drawback of the circuit disclosed in the aforementioned U.S. Pat. No. 4,704,549, as well as other known circuits, is the signal noise that sometimes arises because of the switching of the transistors used in such circuits.